Durability sustained load testing

Many types of tests for evaluating and/or predicting the durability of metal-to-metal adhesive bonds are known in the literature. In our laboratory we have made extensive use of the sustained load overlap shear test at high temperature and high humidity for this purpose. As a result of more than 10 years of evaluation we are able to make qualitative correlations of the durability of metal-to-metal adhesive bonds as a function of adhesive type, temperature of cure, type of adherend, type of surface preparation and the amount of load placed on the specimen. 

The method used for testing durability of adhesive bonds was developed at 3M by W.D. Sell (16) and is called "sustained load stress durability". The metal substrates were 2024T-3 clad or bare aluminum alloy, 5052T-4 bare aluminum alloy or 1010 cold rolled steel. The surface preparation for the aluminum alloys was either the "optimized" FPL-etch" or the H PO -anodization process (12). The steel was solvent wiped. If a primer was used, it was cured before application of the adhesive. Film... 

In all cases, more than one specimen was tested for sustained load durability and the legend for the bar-graphs is as follows. The ordinate shows the sustained load applied to the samples while the abscissa shows the days of exposure. A white bar with an arrow at the end indicates no failure of the specimens, while a white crosshatched bar indicates that at least one sample is still in test. A white bar with a blunt end indicates that all of the samples had failed. In this case, the average of the days to failure of all the tested specimens is given. When possible, the degree of cohesive failure is shown at the end of the failed specimen bar-graph. For example, 0.1 C means 10% cohesive failure and 90% apparent adhesion failure. 

The durability test results in the 171 C-film-curing adhesives are shown in Fig. 8. In general, sustained loads of 50% or less of the control strength with these adhesives show them to be exceedingly durable systems. The materials... 

The sustained load that is applied to the bond in relation to the bonds control strength is certainly a critical parameter. Whether or not the load has any effect on the adhesive itself (analogous to stress corrosion cracking of metals) cannot be ascertained from our data. The literature, especially that of Bascom (19) would indicate that it does. This work does indicate that designers of adhesively bonded structure should take durability testing of this sort into account in their design exper imentation. 

The types of stress acting on completed bonds, their orientation to the adhesive, and the rates at which it is applied are important factors in determining the durability of the bond. Sustained loads can cause premature failure in service, even though similar unloaded joints may exhibit adequate strength when tested after aging. Some adhesives break down rapidly... 

The cyclic immersion test and the stressed high-humidity (sustained load durability) test are particularly aggressive and are both discriminating tests for adhesive durability performance. 

The results of the sustained load durability testing and the outdoor weathering studies both give the same order for the relative durabilities of the different adhesive classes and these are summarized in Table III. However, such a ranking of performance should only be taken as a guide. 

Durability test methods lack standardization and are not always representative of realistic service conditions it is therefore essential to develop standard accelerated ageing tests that correspond to actual in-situ environments. In particular, significant effort should be undertaken to study the synergistic effects between environmental factors (moisture, temperature, alkalinity, chemical attacks) and mechanical loads (sustained loads, fatigue) at both the material and structural scales. 

The problem, of course, is to understand the factors which will provide reproducible, strong, and permanent joining for aircraft structure. There has been considerable concern because material specification tests in standards of even three or four years ago, failed to predict the service problems. This has resulted in major programs in the industry to evaluate these factors. We shall review some aspects of this work and present what we believe is a rational approach to the subject of obtaining durable bonds in hot, wet environments under sustained loading. 

Authors from 3M [14] described the testing of two epoxy adhesives for aluminium designed for hem flange bonding and panel bonding. These adhesives showed outstanding sustained load durability in cyclic corrosion tests even if stressed to 40% of their initial shear strengths. 

Since adhesives are plastic materials, they may creep under high loads and therefore the expected static load is a determining factor for the durability. Compared to lap shear strength or tensile strength measured with shouldered test bars, the resistance toward static load is significantly lower. It is recommended to use only in the range of 3% to a maximum 10% of the maximum lap shear strength for the calculation of the sustainable permanent static loads for the use phase. 

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